A relatively recent development in the coating field is the provision of water based coating compositions which are effective, without the aid of externally applied electricity, in forming on metallic surfaces immersed therein organic coatings that increase in thickness or weight the longer the time the surfaces are immersed in the compositions. (For convenience, a coating formed from such a composition is hereafter referred to as "an organic coating which grows with time" or as an "autodeposited coating".) Speaking generally, compositions which are so effective comprise acidic aqueous coating solutions having dispersed therein particles of an organic material such as resin particles. Autodeposited coatings are formed from such compositions as a result of their ability to attack the metal surface and to generate metal ions in amounts which cause the particles to deposit on the surface in a manner such that there is a continuous buildup of organic coating on the surface.
Coatings formed from such compositions are distinctly different from coatings formed by immersing the metallic surfaces in conventional latices, that is, compositions comprising resin particles dispersed in water. The weight or thickness of a coating formed by immersing a metallic surface in a conventional latex is not influenced by the time the surface is immersed in the latex. It is in the main influenced by the amount of resin solids dispersed in the aqueous medium.
Coatings formed from the aforementioned recently developed coating compositions are also distinctly different from coatings formed from earlier known acidic aqueous coating solutions containing dispersed solid resin particles and relatively high amounts of water soluble corrosion inhibitors, such as compounds containing hexavalent chromium. The use of relatively high amounts of corrosion inhibitors in such solutions deters attack of the metallic surface to an extent such that resinous coatings which grow with time are not obtained. Thus, resinous coatings formed by immersing metallic surfaces in such compositions are like those formed from immersing the metallic surfaces in conventional latices in that they do not grow with time.
The use of the recently developed coating compositions which produce coatings which grow with time offers a number of advantages. For example, other factors held constant, they can be used to apply thicker organic coatings to the metallic surface in a shorter period of time and in a one-step operation. Also, the coating thickness can be controlled by varying the immersion time of the metallic surface in the coating composition. In general, coatings which have improved corrosion resistant properties and aesthetic appearance are obtainable. These are but a few of the advantages which flow from the use of said compositions.
While the aqueous acidic coating compositions described above produce excellent coatings, it is frequently desired to increase the corrosion resistance of such applied coatings (as measured, for example, by ASTM B117).
The commonly employed method of increasing the corrosion resistance properties of autodeposited coatings is to contact the wet or unfused applied coating with an aqueous Cr-containing solution. Typically, the aqueous Cr-containing solution can be an aqueous solution of hexavalent chromium, the source of which may be a water or acid soluble chromate or dichromate or CrO.sub.3 ; or an aqueous solution of hexavalent chromium and formaldehyde-reduced forms of hexavalent chromium. For example, see U.S. Pat. Nos.: 3,585,084; 3,592,699; 3,647,567; and 3,795,546.
While the corrosion resistance of the applied autodeposited coatings can be increased by treatment with aqueous Cr-containing solutions, the corrosion resistance obtained will vary from one autodeposited coating to another due to the large variety of resins which can be applied by this method.
In some end use applications it is desired that the resin coated metallic surface have a relatively high degree of surface slip. This is the case in applications wherein the surface of a coated metal part is in movable surface contact with another coated or uncoated metal part and it is desired that the two surfaces slide across one another easily without scratching the coating (e.g. a coated metal track for a sliding door, trunk hinge, door latch, etc.).
One can achieve surface slip in an autodeposited coating by using an aqueous acidic coating solution containing, as the dispersed resin therein, a resin having an inherently high degree of slip (e.g. polytetrafluoroethylene or polyethylene). Unfortunately, the other properties of such autodeposited resins--in particular their corrosion resistance--may not meet the performance criteria for their intended use. Autodeposited coatings of polytetrafluoroethylene and polyethylene display relatively low corrosion resistance by comparison with other types of autodeposited resins. Polytetrafluoroethylene has another drawback in that very high temperatures are required to fuse the applied coating into a continuous film.
It is, therefore, an object of this invention to provide a method of applying an autodeposited coating having a high degree of surface slip while concomitantly providing a high degree of corrosion resistance in the applied coating.